Tuatara: Volume 31, Issue 1, July 1991

Plant Succession and Species Diversity in Vegetation around Lake Pounui, — Southern North Island, New Zealand

Plant Succession and Species Diversity in Vegetation around Lake Pounui,
Southern North Island, New Zealand.

by D.R. McQueen
School of Biological Sciences, Victoria University of Wellington.

Abstract

Forests of Nothofagus solandri var. solandri around the shores of Lake Pounui, West Wairarapa, have been burnt since the 1850s. They were on loess-capped spurs, in a foehn wind influenced climate, of 1500mm annual rainfall. Some sites of fires had, probably, up to four later fires. The resulting mosaic has enabled succession to be studied in seral scrub and forest stands. These were Leptospermum dominated at 14, 30, 50, 70 years, and Nothofagus solandri dominated at 90 and 120 years. A forest of over 200 years was taken as an endpoint.

Soils showed more truncation in younger stands than in older, an effect of greater fire frequency. Soil organic matter increased with stand age. Upper soil pH increased as Leptospermum scoparium became dominant, then dropped under increasingly older Nothofagus solandri. The difference between pH of upper horizons and that of the C horizon suggested that there was less leaching under Leptospermum scoparium that under Nothofagus solandri. Leptospermum scoparium is here considered as achieving facilitation, in both soil and shelter functions.

The stands were studied by the Point Height Intercept method, for foliar density for the stand, (Crown Space %) and for each species (Species % Crown Space). Expressed as vertical profiles, variations in foliar density enabled subaerial competition to be followed through the succession. Leptospermum scoparium, and to a lesser extent, Kunzea ericoides and Erica lusitanica, the first woody dominants were, by 70 years, infiltrated by the more shade-tolerant Nothofagus solandri, dominant at 90 years, with no understorey. Understorey shrubs had been important in younger stands and the same species reappeared at 120 years and beyond. The succession is assigned to Bray's Intermediate model, but with affinites to the Relay Floristic model.

Species richness was high (c 35) in the Leptospermum scoparium dominated stands and fell to its lowest (14) in the 90 year pole stand, regaining values around 23 in the submature and mature forests. Species diversity and equitability, from the Shannon-Wiener index, using Species % Crown Space weighting, was highest in the 14 year and 70 year stands. These two stands had the highest values of Crown Space %, and were both in a transitional state, the 14 year stand from grassland to scrub, and the 70 year stand from scrub to young forest. Species diversity was lowest during the succession in stands dominated by Nothofagus solandri. There was a trend similar in this succession to that towards deciduous mediterranean Quercus forest, in that diversity was highest in seral stages and dropped at the climax forest. Both the Lake Pounui and mediterranean forests are in climates with seasonal moisture stress, and on poor soils. Their successions show trends in diversity contrary to those found in cool humid climates on young soils.

Introduction

The indigenous shrubs or small trees Leptospermum scoparium, and Kunzea ericoides are often involved in succession after fire, on temperate forest sites in New Zealand. Widespread land reclearing, after 1945 until the mid eighties, has reduced the areas of such scrub and low forest and near centres of population firewood extraction continues.

The aims of this study on sites of burnt Nothofagus solandri var solandri forest, all incorporating interactions in the vegetation / soil system were:

1:

To describe plant succession from a series of fire-originated stands, towards forest.

2:

To compare this succession with appropriate successional models.

3:

To determine variation in species richness, diversity and equitability of vascular species through the succession.

Field work was done around Lake Pounui in 1973 and 1979 by senior students of the Botany Department of Victoria University of Wellington, who also did the preliminary computations.

Previous Work

After destruction of temperate New Zealand forests by fire, succession from scrub back to forest is very often started by the ericoid leptophyll (Specht, 1979) shrubs, Leptospermum scoparium and Kunzea ericoides. Leptospermum scoparium is more common on lower fertility soils and Kunzea ericoides on higher fertility soils (Druce, 1957; Burroughs, 1973). Mirams (1957), however, documented a succession initiated by Leptospermum scoparium which is replaced by the longer-lived Kunzea ericoides, possibly as a result of forest floor nutrient increase from the Leptospermum scoparium litter.

Although most obvious in secondary vegetation, both plants grow too in apparently stable and self-perpetuating stands. Yin Ronghua et al. (1984) list such stands. Leptospermum scoparium is found on lake edges and in depression mires, demonstrating a tolerance for sustained waterlogging. It also grows in stable stands on ombrogenous mires and on very infertile soils, demonstrating tolerance to unbalanced nutrition. Kunzea ericoides formed prehistoric vegetation, on rain shadow plains of the South Island (Molloy et al. 1963). There is evidence, (Burrell, 1965) that extant stands of Kunzea ericoides in drier parts of western and central Otago are self perpetuating under semi-arid conditions. Both shrubs also form natural seral communities on terraces of aggrading lowland rivers within the forest areas of New Zealand. After flood destruction these seres often restart with either species, apparently dependent on closeness of seed-bearing plants (Adams, 1986).

Both Leptospermum scoparium and Kunzea ericoides have minute and copious seeds with over 100 per capsule for Leptospermum scoparium (Mohan et al. 1984). They are easily transported by wind. Dispersal from the range of self-perpetuating stands would explain the ubiquity of both species on burnt forest sites.

Stands of both species are frequently found initiating succession on areas from which forest has been absent for some time. This is the case either on land which has been grassed for farming, then abandoned because of soil fertility decline (Blaschke, 1988), or in areas where there has been more than one fire since the original forest (Druce, 1957), and where distance from seed trees, and soil degradation, prevent direct re-establishment of forest.

There have been several detailed accounts of succession on former forest sites, involving Leptospermum scoparium and Kunzea ericoides. Druce (1957) described Leptospermum scoparium dominating the first four decades on repeatedly burnt sites of Nothofagus truncata with later forest being dominated by Weinmannia racemosa. In the same area, on richer soils, Kunzea ericoides was seral after destruction of a mixed forest. Wassilieff (1980) described succession following fire in 1893 on tephra overlying calcareous sediments, north of Napier. Here Leptospermum scoparium went from pure dominance, to co-dominance with Kunzea ericoides in 22 years. The latter became dominant 45 years after the fire. The demise of Leptospermum scoparium was attributed to insect damage. Wassilieff (1983) described in detail the successional pathways on a range of soils in the Marlborough Sounds area. On soil parent materials chemically similar to those of Druce (1957) she found a similar fertility relation and chronology of succession, between Leptospermum scoparium and Kunzea ericoides, over a wider range of local climates.

Successional Models

The studies outlined in the preceding section all carry the assumption of succession as described by Clements (1916) in which one species, or group of species, follows another in the succession and where the preceding species alters the environment to the advantage of the following species. This model is now called Classical (Bray, 1989).

It is only recently that New Zealand ecologists have attempted to fit local species composition data to later North American models proposed:

1:

The Initial Floristic Composition model of Egler (1954)

2:

The Relay Succession model of Drury and Nisbet (1973), which is an amplification of Clement's principles.

3:

The Facilitation and Inhibition models of Connell and Slatyer (1977)

4:

The Resource Ratio model of Tilman (1985)

Bray (1989) presented a concise review of successional models 1-3 (above), and tested them against data from secondary Leptospermum scoparium-Kunzea ericoides vegetation, leading to podocarp forest, in high precipitation climates of N.W. Nelson. His results came from areas where seed-bearing podocarps are close to the studied seral stands and where bird populations are apparently sufficient in variety and number to carry tree seeds into successional vegetation. He found that all the successions (his Figs. 3–7) did not fit models 1-3 (above). He classified all the successions as Intermediate, a new term. In his Intermediate model there is a mixture of species positions in successions, some according with the Relay Floristic model and some with the Initial Floristic Composition model. Bray's criterion for the Intermediate model being applicable is “a sequence of species arrivals, with the latest one establishing before the demise of the earliest” (Bray, 1989).

There is variously Facilitation and Inhibition (Connel and Slatyer 1977) in Bray's successions. Inhibition is shown, apparently, by early arrivals of tree species, with their disappearance soon after and a later renewed continued establishment of the same species.

Bray's work was not the first attempt to fit New Zealand succesions to overseas models. Two previous studies on succession involving manuka and kanuka were those of Wassilieff (1982) and Adams (1986). Neither were able to assign succession to Initial Floristic composition or Relay Floristic models. Their results suggest too the Intermediate model of Bray (1989).

Mark et al. (1989), described succession in a superhumid climate, on a series of landslides of increasing age, on a slope uniform in substrate, aspect and slope. The observations had been over a period of 24 years and earlier chronology was derived from ring counts. They assigned the succession to the Relay Floristic model, with facilitation by early Leptospermum scoparium in establishment of forest dominated by Nothofagus menziesii, with Weinmannia racemosa and smaller amounts of Nothofagus solandri var. cliffortioides and Podocarpus hallii. Mark et al. took account of the resource-ratio model of Tilman (1985) as incorporating a temporal gradient of availability of limiting resources, and they postulated that nitrogen and light levels could be relevant to the succession they described.

Species Diversity

Generalisations about species diversity abound. In relation to environment, species diversity is believed to be low in extreme conditions and high in optimum conditions. In relation to succession Whittaker (1974), believed that diversity is highest in a climax forest while Margalef (in Odum, 1971) postulated that diversity would peak in mid-succession towards forest.

“Species richness”, the number of species in a sample area, is the simplest expression of diversity, but does not take into account variations in plant abundance in each sample. Such variations can include monospecific dominance, a common situation in development of forest stands and one which either greatly reduces the importance of other plants, or excludes them. Dominance can also be shared by several species, both in the canopy, and below. This can be a common condition early in a succession. This variability in sharing of resources is expressed by using a quantity-weighted index of diversity.

Whittaker (1972) summarised the development of quantity-weighted diversity indices including the Shannon-Wiener information index (H′), chosen for the present study. H′ functions as a measure of equitability between the species in a stand of vegetation. When H′ is high, there is sharing between many co-dominant species of the space available and if H′ is low, one, or very few, species dominate the stand.

In New Zealand there have been few studies published on species diversity in terrestrial vegetation. Three of these studies use simple species richness as an index.

Williams (1982), using simple species counts of insects and vascular plants found for some off-shore islands of New Zealand, that biogeographical models (MacArthur and Wilson 1967) were satisfied for distance from New Zealand, island area and altitude.

Wilson and Sykes (1988) in indigenous and adventive vegetation found that invasion of adventives had little effect on species diversity in indigenous and adventive vegetation. Mark et al. (1989) described a succession of Nothofagus menziesii forest, in which the number of vascular species peaked in mature forest, agreeing with Whittaker (1974). Wilson, Lee and Mark (1990) used quanitity-weighted diversity to show differences in species diversity between vegetation on ultramafic and schist soils in S.W. New Zealand.

Description of the Study Area

Topography, Geology and Climate:

Lake Pounui (NZMS260:R27/ 867835) is at c. 15m altitude, and surrounded on three sides by low rolling hills and plateaux, up to 120m altitude. These surfaces are slightly warped Pliocene to Pleistocene silts and gravels derived from the uplift of the axial Rimutaka and Tararua mountains. Soils on flat to convex slopes show evidence of one or more periods of loess deposition (Kamp and Vucetich 1982)

The Rimutaka mountains, to the N.W. of Lake Pounui, rise to 1000m, and cause foehn winds, whose drying effects create a greater seasonal moisture deficit than is expressed by the annual 1576mm of rain, recorded as a mean over 52 years, at Waiorongomai, 10 km to the north of Lake Pounui. The mountains to the northwest of Lake Pounui are c. 300m higher than those northwest of Waiorongomai, so the rainfall there may be higher than at Lake Pounui. At Waiorongomai winter rainfall peaks at 357mm, in June. The driest month is February, with 242mm. The warmest month, February, has a mean maximum of 21.7°C, and the coldest, July has a mean minimum of 4.6°C. Ground temperatures show a mean of 28.7 days of frost, from March to November. [Temperatures for an 11 year period. All climate data comes from N.Z. Meteorological Service (1981)].

Forest Vegetation:

The remaining forest is extensive to the north and west of Lake Pounui. On plateaux and ridges Nothofagus solandri var. solandri (black beech) forms pure stands, with a sparse understorey. Lower down the slopes Nothofagus truncata (hard beech) becomes increasingly important. At the foot of slopes are scattered podocarps, Prumnopitys taxifolia and Dacrydium cupressinum. On valley floors the soil is far moister than on higher land and is of fine gravel interbedded with buried organic soils. Here there is a swamp forest with Syzygium maire and a few Dacrydium dacrydioides left after logging in the 1940s (P.F. Jenkins, pers.comm.)

On the plateaux to the north and east of the lake and on their slopes down to the lake, most forest was burnt, possibly in early farming attempts. The first farming in the Wairarapa valley dates from 1844 (Bagnall 1976). Fire associated
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with pig hunting (Donald Cameron pers.comm.) has been repeated around and above the lake and has resulted in a mosaic of successively younger seral vegetation. The dominant plant in this mosaic of vegetation is, in younger stands, Leptospermum scoparium. Older stands show increasing dominance by Nothofagus solandri var. solandri, a result of the proximity to many of the stands of seedbearing Nothofagus solandri var. solandri.

Methods

Nomenclature

Field

Stand physiognomy was identified from aerial photographs, and ground inspection gave detail on height, species composition and possible successional order of stands. Sampling was restricted to the upper ridges and spurs, on convex or flat slopes, to sample only the soils on reasonably intact loess. Within such a stratification, the locations of individual stands were randomly selected to give representative coverage of all stand sizes.

Quadrats of 3m × 12m were laid out in scrub across the slope. Vegetation and site data were collected within rectangular quadrats laid across slope, following Godron (1968). The quadrats were 3m × 12m in scrub, and 5m × 20m in forest. Around the quadrats extra areas were searched for species presence, giving totals of 112m2 in scrub and 216m2 in forest.

Within each quadrat 100 PHI (Point - Height Intercept) stations were laid out systematically and recorded, following Park (1973). Contacts up to 2m height, were recorded against one corner of a 1cm3 metal pole at every 10cm vertically. Above 2m, optical intercepts were recorded by a gimbal-hung crosswire sight, their heights being measured by hypsometer and measured baseline. These higher intercepts were at height intervals of 20cm up to 4m height and 1m above 4m.

Soil pits were dug at each site and samples collected for analysis.

In each scrub stand, discs were taken from ten larger Leptospermum scoparium plants and growth rings counted to age the plants, and to ascertain to the approximate date of initiation of woody growth. In Nothofagus solandri var. solandri stands 5mm diameter cores were extracted from ten trees. This aging was less reliable as an estimate of stand initiation date, as it was not known whether the earlier fires were followed directly by Nothofagus solandri var. solandri or successionally through Leptospermum. Nothofagus does not seed every year (Poole, 1987), so scrub stands may occupy the site for up to 40 years (Druce, 1957) in similar conditions to those at Lake Pounui.

Laboratory

The PHI results were analysed on the programme “PHI” (Hall and Frost, 1984). One of the first operations in “PHI” is to render all height class readings to a uniform scale, based on the minimum height interval, following Park (1973). This process may appear to give an exaggerated measure of species intercepts higher in the vegetation, but has to be accepted as a limitation of the technique.

The number of intercepts per height interval were extracted from the PHI output and plotted by “Cricket Graph” on a MacIntosh 512 computer to give vertical profiles for the main species of the succession.

Crown space percent is defined (Park, 1973), as:
where vegetation space is the space beneath the uppermost intercepts of the stand sampled. CS% is an estimate of density of foliage.

Species Percent Crown Space, (S%CS) (Park, 1973) was also extracted from the PHI output:

S%CS is an estimate of quantity of each species in a sample and was used as the weighting factor for calculation of the Shannon-Wiener quantity weighted diversity index, H′:
The modification
was suggested by Shirley Pledger, Mathematics Department, V.U.W., (pers. comm.) as a means of reducing the influence of species simply present and not encountered by PHI.

Diversity terminology

Following Williams et al. (1969), “Species Richness” will be used for total species content of each sample, S in the equation above. Diversity will refer to the quantity-weighted diversity index, H′, above.

Soils

Soil pits were dug at each quadrat site and described (Taylor and Pohlen 1970), including estimates of gravel and larger mineral particles.

Bag samples were collected:

1)

for sieving to estimate the percentage of material > 2mm

2)

to determine pH

3)

to determine the organic matter by loss on ignition.

Undisturbed core samples were collected from youngest, medium-age and oldest quadrats, for determination of macroporosity % volume (>60u diameter). All the above analyses were done by methods in Staff of Soil Bureau (1972).

Results

Stand Description and Fire History

Table 1 summarises details of quadrat sites and composition. The following descriptions add detail on special features of the stands and an outline of fire history. The 200, 90, 70 and 50 year stands were on the relatively weakly dissected upper slope, above the southeast shore of the lake. Their fire history is interrelated because
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of ease of fire movement along a slope exposed to both northwest and southeast winds. The 120 and 30 year stands above the northwest shore were on spurs, still wind exposed, but separated by several sharp gullies.

200 year Nothofagus solandri var. solandri forest. This forest is assumed to be part of the original forest. Its age is greater than 200 years, but heartrot prevented full aging. The only tree seedlings are those of Nothofagus solandri var. solandri which appears self-perpetuating. This stand, being on a spur, is prone to wind disturbance of the litter, leaving quite large patches of exposed topsoil and localised concentrations of litter, especially in the angles of superficial tree roots.

120 year Nothofagus solandri var. solandri forest: This forest possibly originated from fire soon after the first farming contact in 1844 (Bagnall, 1976).

90 year Nothofagus solandri var. solandri pole stand: This stand is on upper slopes above that studied near the lake shore by Bagnall (1972) and appears continuous with it. He aged, from stem discs, trees which started growth in 1891 (88 years old at 1979). The stand studied in the present paper contained very suppressed Leptospermum scoparium, which gave the 90 year age. It is probable that the upper slope stand did not regenerate directly to Nothofagus solandri var. solandri, but that exposure and distance from the older seed trees existing near the lake shore caused development of a scrub stand which was then invaded by Nothofagus solandri var. solandri.

70 year stand: Leptospermum scoparium, Nothofagus solandri var. solandri. This stand probably originated from a second fire in the 90 year old stand. It was on the same face above the lake and 900m downwind, in a prevailing wind. It was cleared for farming within a few years of being studied.

50 year stand: Leptospermum scoparium. Abutting the 90 year stand with a sharp margin, this scrub stand originated after probably three fires, one of which orginated the 90 year stand, one caused the 70 year stand and one on the site of the present stand.

30 year stand: Leptospermum scoparium, Erica lusitanica. Closer to the public road than other stands, the species composition of this stand reflects this in the presence of Erica lusitanica, which grows along cuttings on the road. The absence of any remnant trees suggests there have been several fires, but distance from any clear burn margins does not allow any estimate of their number.

70 year stand: Leptospermum scoparium, Nothofagus solandri var. solandri. This stand probably originated from a second fire in the 90 year old stand. It was on the same face above the lake and 900m downwind, in a prevailing wind. It was cleared for farming within a few years of being studied.

50 year stand: Leptospermum scoparium. Abutting the 90 year stand with a sharp margin, this scrub stand originated after probably three fires, one of which orginated the 90 year stand, one caused the 70 year stand and one on the site of the present stand.

30 year stand: Leptospermum scoparium, Erica lusitanica. Closer to the public road than other stands, the species composition of this stand reflects this in the presence of Erica lusitanica, which grows along cuttings on the road. The absence of any remnant trees suggests there have been several fires, but distance from any clear burn margins does not allow any estimate of their number.

14 year stand: This quadrat was located on a recently abandoned corner of a paddock. The land had been cleared of the 50 year scrub which it adjoins, then grassed, but abandoned as pasture when a section was cut off by a straight fence. It may
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have been burnt four times. Sheltered by the adjacent older Leptospermum it would have been a favoured sheep “camp’, and is likely to have had an initially more fertile soil. With pasture extension it has been destroyed.

Animal damage:

Stands from 50 to 120 years showed some sign of animal damage, particularly to Pseudopanax arboreus and larger leaf Coprosma spp. Such damage was ascribed to possum (Trichosurus vulpecula) and red deer (Cervuus elephus).

Soils

The analyses performed were limited, but can be interpreted in terms of some general principles about soil changes following fire. Fires have several main effects on soils (Barbour et al., 1980). The upper, richer, horizons lose nitrogen and potassium by volatilization and other nutrient elements become more soluble, making them at first more available to plants, but also more liable to rain water leaching. Neary et al. (1978) reported substantial losses of nitrogen, phosphorus and potassium into streams, from felled and burnt mixed podocarp-beech forest in Westland. Physically, fire can burn the humus layer and leave the soil below disaggregated by heat. This renders it less porous and prone to erosion until plant cover is restored. This erosion also causes truncation of the mineral soil profile, leaving the lower less fertile, with less porous horizons for plant growth. Repeated fire on the same site will continue the soil deterioration process.

Coarse mineral fraction:

The field result of coarse material estimates and the laboratory sieving, for the >2mm fraction, were combined, as a mean percentage of total coarse fraction. As the soils are from a loess cap, it would be expected that an intact profile, under forest, would have the least coarse material in its upper mineral horizons. With truncation of the soil profile following an increasing number of fires, the underlying stonier soil formed from colluvium would be further exposed. There is a slightly significant negative correlation between stand age and percentage of >2mm mineral particles (r=0.67, P=0.10-0.05). The uppermost mineral horizon under younger vegetation does have more coarse material than under older vegetation, demonstrating truncation of profiles with greater fire frequency.

Loss on ignition:

This approximation to organic carbon content (Duchaufour, 1965), has the disadvantage that, as well as carbon being driven off by the 800°C temperature used, the water of constitution of the clays are lost. However the soils analysed were uniformly weakly weathered, with a dominant and consistent silt loam texture, so that the clay water would be relatively uniform.

Fig. 1A shows a significant correlation between stand age and the mean loss on ignition down the profiles (r=0.83, P=0.05-0.01). An aberrantly high point at 14 years, is from the youngest Leptospermum scoparium stand, on a former pasture and likely to have a residual organic matter enrichment in the topsoil.

Soil Acidity:

Fig. 1B shows pH changes during succession. The horizons sampled in each case were the uppermost rooted organic horizons, the Ai from Leptospermum dominated stands, and the H from Nothofagus stands from 90 years on. In the first 50 years pH rises to 5.7, then with the increase in Nothofagus from 70 years, pH decreases to its lowest value, 4, in the H horizon of the 120 year Nothofagus stand. These pH values may be linked to the litter pH of the two main species involved:
page 51Leptospermum scoparium litter, from the 120 year stand had a pH of 4.0. The H horizon of the mature stand has a higher pH, possibly because of the more open, wind-disturbed floor and consequent more active mineralisation of the organic layers.

Nothofagus solandri var. solandri is a species of moderately low nutrient return potential, which can be approximated by litter pH. Its litter has 91 m.eq.% alkalinity. This compares with 115 m.eq% from Vitex lucens, which grows on fertile soils and forms a null humus and Agathis australis with a litter alkalinity of 50 m.eq.%, forming a very acid mor humus, and podsolising strongly (Staff of Soil Bureau, 1968). In the present study, the pH of the H horizon under Nothofagus solandri var. solandri is except for the mature stand, in the range of 3.8 to 4.4. Such a range is comparable with the pH of H horizons beneath other soil leaching trees, pH 4.0 for Agathis austrlis, 4.1 for Nothofagus truncata (Staff of Soil Bureau 1968).

The difference in pH between the top organic horizon and the C horizon has been used (Fig. 2) as an indicator of leaching when the difference is negative and of enrichment when it is positive. It is positive in the four younger stands, still dominated by Leptospermum (Table 1, Fig. 3). With increasing Nothofagus the pH difference becomes negative, indicating leaching. This trend in soil leaching with time cannot be interpreted as a direct relation with time. In the present case, the older are the stands of vegetation, the fewer fires are likely, with consequently less soil impoverishment.

Confirmation of the presence of more type humus formation is found in the correlation between pH and loss on ignition of the uppermost organic horizons (r=0.84, P=0.05-0.01). The more the organic matter has accumulated, the lower is the pH.

The Succession

Regeneration Mechanisms:

Of the trees and shrubs in this succession the only one known to resprout after stand damage is Weinmannia racemosa (Druce, 1957). The rest depend on seed reproduction. Of the trees and shrubs, only Pseudopanax arboreus has a seed adapted to bird carriage. The others have varying degrees of seed mobilty by wind, except Nothofagus solandri var. solandri. In common with other Nothofagus not only does this species have a heavy unwinged seed, capable of wind transport only for very short distances (McQueen, 1951, Wardle, 1984), it also seeds irregularly, in mast years, at times over 10 years apart. Regeneration of Nothofagus can also depend on any surviving advance growth on the forest floor (Poole, 1955). There is little likelihood, however, of survival of young Nothofagus in a scrub stand, where the main fuel plants are at the same level as the Nothofagus saplings.

Thus in the youngest stand (50 years) containing Nothofagus solandri var solandri, it is likely, because of repeated fires, that this tree entered the sere after Leptospermum scoparium, rather than initially, because of the absence of seed trees. Such an entry sequence is described by Bruce (1957), for Nothofagus truncata on areas burnt several times. The similarity of the present case to his example can help confirm the idea that the younger stands at Lake Pounui have been burnt more than once.

The taller, monopodial, shrubs and trees involved in this succession show varying reactions to light conditions in developing forest stands. There are three leptophyll (Specht 1979) shrubs to small trees which do not persist under any form of overhead cover: Leptospermum scoparium, Kunzea ericoides and Erica lusitanica (adventive). One nanophyll tree, Nothofagus solandri var. solandri, is more shade tolerant, as are the three mesophyll trees: Pseudopanax arboreus, Knightia excelsa and Weinmannia racemosa. Pseudopanax arboreus is unlikely to be aggressive under the existing possum and deer browsing conditions. Knightia excelsa and Weinmannia racemosa are usually
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found in more mesic soil conditions, so the subsidiary role of these three species in succession here may equally be an expression of climate intolerance.

Species and Structural Changes in the Succession:

Fig. 3 shows that at 14 years grasses and herbs of open country are still important, but Leptospermum scoparium, (Fig. 4) forms a low dense canopy. Weinmannia racemosa is present, in small quantity and continues through the succession as a minor component. Coprosma rhamnoides and Dianella intermedia persist throughout the succession. The presence for the same time span as Leptospermum scoparium, of the understorey monocotyledon Leptospermum australe, also found in mires, can be related to the low macroporosity (Table 1:15% in the A horizon at 70 years) of the truncated soil profiles of the younger stands. The B horizon of intact loessal soils under Nothofagus truncata, can have macroporosity as low as 5% (Staff of Soil Bureau 1968).

At 30 years the shrub canopy, of Leptospermum scoparium and some Kunzea ericoides and Erica lusitanica is dense (Fig. 4) and only occasional Hypochoeris radicata remains from the old pasture. Two moderately shade tolerant trees are present at this early stage of succession. The first is Weinmannia racemosa, possibly resprouted. It was present in small quantity at 14 years, and persists until the submature Nothofagus solandri var. solandri forest at 120 years. Two small trees also appear spasmodically through the succession, with their first prominence at 30 years. The first, Kunzea ericoides is a seral shrub of sites more fertile than those dominated by Leptospermum scoparium (Druce, 1957, Burrows, 1973). The second, Pseudopanax arboreus frequently succeeds Leptospermum scoparium in moister edaphic conditions (Wassilieff, 1983; Blaschke, 1988), than those of upper slopes around Lake Pounui.

The occurrence of the adventive, Erica lusitanica, reflects the closeness of the 30 year sample to roadside volunteer patches. It does not recur in the succession. Two shrubs of forest understorey, Leucopogon fasciculatus and Cyathodes juniperina appear as important components at 30 years. They persist (Fig. 3), in varying quantities throughout the succession.

At 50 years Nothofagus solandri var. solandri makes its first appearance, above and within the rising Leptospermum scoparium canopy and with its foliage inserted in gaps in the shrub canopy (Fig. 4). The establishment of Nothofagus solandri var. solandri has depended on provision of seed, possibly blown upslope from mature trees fringing the lake. Also, the soil condition at 50 years, with 14% organic matter in the A horizon and a fermenting litter 1.5cm deep, are amenable to Nothofagus establishment. Thus the Leptospermum scoparium cover has facilitated (sensu Connell and Slatyer, 1977) the establishment of the forest.

In the 50 year stand all the main species of the mature forest are present, although the community is still dominated by shrubs. Fig. 4 shows the Nothofagus solandri var. solandri foliage infiltrating the scrub canopy at all heights. Nothofagus solandri var. solandri has sinuous, long juvenile branches which aid such infiltration.

By 90 years the Nothofagus solandri var. solandri canopy is dense and close to the ground (Fig. 5) and borne on closely spaced trunks. Light is perceptibly reduced. In relation to both younger and older stands, there is a reduction in species richness and in their quantities. It is in this stand that the main early seral shrub Leptospermum scoparium was last found as a few almost leafless suppressed stems.

The 120 year stand shows that with self-thinning and the further raising of the canopy from the ground (Fig. 5) there is a corresponding increase in two understorey plants Cyathodes juniperina and Leucopogon fasciculatus. There is also the reappearance of Gahnia pauciflora a plant which was found earlier in the upper layers of the 30 years stand.

The 200 year stand shows a canopy base about 7m clear of the ground (Fig. 5). The wind redistribution of litter, together with root competition in a shallow upperslope soil, would contribute to the spareseness of the understorey (S%CS Σ (understorey plants combined) = 0.84%).

Variations in CS% During the Succession

CS% shows its highest value in the 14 year stand of 62% (Table 1). This low (0.7m) vegetation was not yet tall nor dense enough for much shading out of lower foliage of Leptospermum scoparium, as well as having Dianella nigra at 15% CS%S. CS% then declines with age, with progressive rising of the scrub canopy, until at 70 years CS% reaches a second peak. In this stand (Fig. 8A) there is a dense canopy of leptophyll light demanding shrubs, and as well there is 34.5% S%CS of the more shade tolerant Nothofagus solandri var. solandri, both mingled with the shrub canopy, and below it.

When Nothofagus solandri var. solandri dominates, from 90 years, CS% remains low in the two pole stands, then rises, to 30% in the mature forest, whose Nothofagus solandri var. solandri trees have a deep and extensive canopy.

Species Diversity During The Succession

Species Richness:

In the present study, species richness is defined as the total number of vascular taxa, rooted in the soil. Fig. 6A shows this is variable during the succession. In the early seral stages, when Leptospermum scoparium dominates, there is first an increase in species richness, from the 14 year, low and dense stand, where (Fig. 4) there is little space beneath the Leptospermum scoparium canopy. As this canopy rises, species richness increases to its highest, to 35 taxa by 30 years. This level is maintained through to the 70 year stand, with an increasing proportion of sub canopy and canopy space being occupied by Nothofagus solandri var. solandri (Fig. 4). This taxon, at 90 years then dominates the space, (Fig. 5), with its more shade tolerant foliage, so much so that few plants can tolerate the dense shade. At this 90 year dense pole stand stage, species richness is at its lowest — 14 taxa. In the submature Nothofagus solandri var. solandri stand of 120 years, the opening out, by self-thinning, has allowed more light to reach the floor and species richness rises to 23. A similar value is found in the mature forest. This value is considerably lower than the 52 vascular species recorded by Mark et al. (1989) in mature Nothofagus menziesii forest, under a superhumid climate. This high species richness found in the succession studied led them to agree with Whittaker (1974) that a climax forest has the highest species diversity in a succession. Such is not the case in the Lake Pounui succession. There are several possible causes

1)

the soil leaching regime associated with Nothofagus solandri var. solandri limits the number of species capable of occupying the site;

2)

the climate is drier, and edaphic conditions on the spurs aggravate this effect on plant growth;

3)

the effects of browsing may have eliminated some species, particularly mesophyll shrubs.

Species Diversity and Equitability

The Shannon-Wiener index of diversity, (Fig. 6A) is calculated from the number of taxa, and the PHI species frequency, S%CS. Antilog H′ is also a measure of equitability, or degree of sharing of subaerial niche space, by the various taxa
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composing a sample of vegetation. Fig. 6A shows that during the succession, the comparison of variations between the line for S, and that for H′ can demonstrate variations in degrees of dominance by the taxa involved.

At 14 years antilog H′ is slightly above 2, an expression of some sharing of dominance of Leptospermum scoparium with notably, Coprosma rhamnoides and Dianella nigra.

By 30 years antilog H′ is 1.73. The canopy of Leptospermum scoparium, Kunzea ericoides and Erica lusitanica, (combined on Fig. 4 because of morphological and ecological similarity), is dense and has little space beneath for development of an understorey. Leucopogon fasciculatus, Cyathodes juniperina and Gahnia pauciflora also contribute to this canopy, which is 2.5m high. Beneath it only Uncinia banksii and Lepidosperma australe exceed 1% S%CS each.

At 50 years the lower boundary of the Leptospermum scoparium canopy is now 1m above ground level. Nothofagus solandri var. solandri has now entered the community, and as Leucopogon fasciculatus and Cyathodes juniperina have S%CS values each over 10%. Antilog H′ has slightly increased its value on that of the 30 year stand, to 1.82.

At 70 years antilog H′ then rises steeply, to 2.85. Fig. 4 shows that the S%CS value of Nothofagus solandri var. solandri is double that of Leptospermum scoparium. As well 10 other species have S%CS values >1% and of these, 3 are >5% in S%CS.

By 90 years antilog H′ has fallen to 0.75. Fig. 5 shows a dense canopy of Nothofagus solandri var. solandri. It has space beneath it. but this canopy is so dense that little light reaches the ground. Foliage intercepts reach a maximum of 30 at any one level.

At 120 years antilog H′ rises slightly to 0.98, the Nothofagus solandri var solandri canopy is taller than at 90 years, and is also deeper. Foliage intercepts here only reach a maximum of 17 in any one level. This is an expression of a more open canopy than at 90 years, here reflected by an understorey of Cyathodes juniperina and Leucopogon fasciculatus totalling 18.6 S%CS.

In the mature stand antilog H′ drops to 0.71, the lowest value in the succession. The canopy here is the largest encountered in this study and it has weighted equitability strongly towards one species, Nothofagus solandri var. solandri. This species has a maximum of 45 intercepts in any level of observation. Most of this canopy is more than 7m above the ground. There are seven species in the understorey, but their summed S%CS value is <1%. This sparsity of understorey can be ascribed to three factors,

1:

the presence of browsing mammals, red deer and possum,

2:

the strong wind disturbance of litter and arrested and localised pedogenesis;

3:

root competition, particularly for water on this upper slope stand.

Species Richness and Diversity in Relation to Stand Structure

Crown Space Percent is weakly postively correlated with antilog H′ (r = 0.68, P = 0.10 − 0.05), and not correlated with species richness (r = 0.3). Fig. 6B shows the relation between antilog H′ and CS%, as a grouping of the stands by dominance. The Nothofagus solandri var. solandri stands at 90, 120 and 200 years share low values of CS% and antilog H′. The low equitability is a consequence of monospecific dominance. The low CS% is a function of the single-stemmed self-pruning nature of the dominant tree.

Higher equitability is found in Leptospermum scoparium dominated stands, where there is more sharing of dominance by other woody species. Younger stands have higher equitability: at 30 years CS% is high, in a dense young stand, at 50 years there is some canopy opening of the aging Leptospermum scoparium, but at 70
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years the canopy has reclosed, with the growth of Nothofagus solandri var. solandri. This 70 year stand is in transition from scrub to forest, hence its density, shown by its high CS%, and its high degree of sharing between different species of varying light tolerances. At 14 years, both equitability and CS% are high. This stand, like that of 70 years is in transition from one type of vegetation to another. The 14 year stand differs from all the others in its admixture of shrubs and herbs. Reiners et al. (1971), show two similar peaks of diversity at transitional stages in a primary succession and an early drop in equitability at a monospecific dominance stage.

Species Richness and Diversity in Relation to Soil Factors:

Species richness is positively and significantly correlated with pH of the upper organic, rooted, horizon, (r = 0.80, P = 0.05 − 0.1). Such a correlation accords with general theory of species diversity, that the more difficult are the conditions the lower is the species richness (Whittaker, 1975).

Diversity, antilog H′, also showed a weak positive correlation with pH of the upper soil, and with the indicator of leaching, (pH difference between organic and C horizons). In both cases r = 0.75 (P = 0.10 − 0.05). These are indications worthy of further work, with more samples.

Discussion and Conclusions

Soil relations: Nothofagus solandri var. solandri dominates in high rainfall areas on drier sites on upper slopes and plateaux (Wardle, 1984, Blaschke, 1988), where more mesic plants with larger and softer leaves, likely to be mull-formers, cannot survive. Nothofagus solandri var. solandri can apparently perpetuate a leaching regime if undisturbed. After disturbance by fire on a Nothofagus solandri var. solandri site Leptospermum scoparium produces a less acid litter. Such behaviour on leached soils is parallel to that postulated for Betula, in remelanising podsolised soils beneath Calluna vulgaris, on former Pinus silvestris sites (Dimbleby, 1952), over a far longer timespan than is the case in the present study.

The rise of organic carbon with increasing stand age parallels the increase in soil organic matter recorded in chronosequence studies on primary successions (e.g. Crocker and Major, 1955). However, in the present study there is no continual temporal pedogensis, rather a series of younger starting points for soil reconstitution, on soils increasingly impoverished and truncated by repetition of fires.

Succession Models: The succession described in this paper can be compared with the results of Bray (1989). His “Old river terrace-manuka series” has community composition approximating that of the present study, in the pre-forest stages. The majority of trees in Bray's succession, all podocarps, enter the Leptospermum scoparium scrub about 20 years earlier than does Nothofagus solandri var. solandri at Lake Pounui. Trees entering Bray's succession earlier than 30 years are apparently suppressed by heavy shade, but reappear later, when conditions are more appropriate. This behaviour is interpreted by Bray as inhibition (Connell and Slatyer 1977). Similar behaviour is shown in the present study at 90 years, when the dense pole stand of Nothofagus solandri var. solandri not only shades out pioneer, light-demanding shrubs, but also reduces the quantity of shade tolerant shrubs to 1.2% SP%CS. These shrubs are usually capable of forming the understorey in more open, older forests. Although this is inhibition of the forest understorey, it cannot be considered as such for the dominating tree, Nothofagus solandri var. solandri. There is certainly facilitation in soil reconstitution, as judged by organic matter accretion (Fig. 1A) afforded by the early growth of Leptospermum scoparium. Facilitation was also postulated in succession from Leptospermum scoparium to Nothofagus menziesii forest, (Mark et al. 1989).

The present study differs from that of Bray (1989) in several respects:

1:

The climate in Bray's sites is wetter and the original forests near his study sites were richer in species, providing a greater variety of seeds able to establish in successions on moister soils.

2:

Bray's entry ages were determined by repeated observation over a period of years, and by coring established plants and there is thus a continuity in time. His entry ages are therefore more reliable than in the present study, based on a series of samples spatially separated and representing the effects of a varying number of fires at each site.

However, the similarity between Bray's results and the present study is strong enough to assign the Lake Pounui succession to his Intermediate model, with many species at Lake Pounui following the criterion set by Bray “that the latest species establishes before the demise of the earlier”. There is still closeness to the Relay Floristic model of Drury and Nisbet (1973). In the Lake Pounui succession the inhibition of understorey species by the dense 90 year old Nothofagus solandri var. solandri (Fig. 3) gives the impression of batons lost during a relay race, so this succession is not completely able to be called a relay.

The resource ration hypothesis of Tilman (1985) is also applicable to the present succession. Here the soil resource appears to be limited, except from the ash-bed effect shortly after fire. Leptospermum scoparium may help restore nutrients, as shown by the pH enrichment indication (Fig. 2). As the soil is reconstituted, Nothofagus solandri var. solandri enters, and becomes a “superior competitor”, (Tilman's term), because its use of light excludes Leptospermum scoparium.

Species Richness And Diversity: The variations in species richness and in diversity in this succession can be related to structural changes of the vegetation through the succession. At the early stage of low woody vegetation, at 30 years, with dominance by few species both S and antilog H′ are low; space and light are limited. The canopy of this early seral vegetation has relatively open foliage, and the species composing it do not tolerate shade. As the lower boundary of this shallow canopy of the vegetation gains height, there is space and light for a greater number of species and for their growth to share the available space, thus increasing equitability, expressed by the quantity - weighted diversity index antilog H′.

The replacement of the shrub community by Nothofagus solandri var. solandri demonstrates the relative tolerance of this tree to moderate shade, in earlier stages in its infiltration of the scrub canopy. When Nothofagus solandri var. solandri dominates, the canopy is deep, excluding any understorey for some decades. It is in such stands that both species richness and equitability reach low levels. There is increased subcanopy space at 120 years and even more so in the mature stand with increases in species richness and diversity, but not as many species as in the 30 to 70 years shrub vegetation.

A close parallel to species richness and diversity trends to the present study is found in Houssard et al. (1980). Their study of succession to mediterranean deciduous Quercus forest was in a climate where moisture can be limiting in summer and where repeated fire and associated erosion have seriously truncated soils. The first tall shrub stage in the work of Houssard et al. shows a peak in species richness and diversity at 50 years. These values then drop steadily, to the Quercus pubescens forest, at 125 years, corresponding to a similar drop when forest is established, in the present study.

There is a marked contrast between the low species richness and diversity of later stage forest in climates and soils producing moisture stress (Houssard et al. and the present study) and the high species richness and diversity of forests in well watered climates on fresh and relatively fertile soils (Reiners et al. 1971, Mark et al. 1989). These differences can confirm that species richness and diversity are in fact associated with more optimal environmental conditions.

Acknowledgements

I wish to thank the third year Botany students of 1973 and 1979 for their field and lab. work. Shirley Pledger, Mathematics Department, Andrew Frost and Dr Graeme Elliot helped with the quantitative and computing parts of this study. Drs Katharine Dickinson, Paul Blaschke and Bruce Sampson gave much scientific and editorial assistance. I also thank Prof. J.B.J. Wells for use of the facilities of the School of Biological Sciences, V.U.W. and Mr Donald Cameron for access to the study area, and accommodation.

Table 1. Details of quadrats used in the study of succession on spurs or plateau tops around Lake Pounui. The heights given are those of the top intercepts from Point Height Intercepts (PHI) and are not the top heights of plants. S%CS = Species % Crown Space, an estimate of relative foliar quantity of each species, derived from PHI.

Fig. 1A. There is a positive correlation (r = 0.83, P = 0.10-0.05) between vegetation age and soil organic matter (mean for each profile), determined by loss on ignition. The youngest stand deviates from this trend as it was from a sheltered piece of former pasture, probably used as a sheep “camp”.

Fig. 1B. Soil pH of the upper horizons: A horizons to 70 years, and H horizons beyond. The youngest four stands are dominated by Leptospermum scoparium and the older three stands by Nothofagus solandri var. solandri.

Fig. 2. The difference in soil pH between the uppermost organically enriched horizons and the C horizons. When positive, this difference suggests enrichment of the upper horizons, when negative, it suggests leaching, as is the case beneath Nothofagus in the three older stands.

Fig. 3. Principal species involved in the sucession after fires at Lake Pounui. All species with > 1% S%CS at any age are included. ========: species > 2% S%CS. ========: species < 2% S%CS, or present in the quadrats without being intercepted. The S%CS values for Nothofagus are included. This diagram shows an Intermediate successional model (Bray, 1989), with some inhibition, but only of understorey species, at 90 years.

Fig. 4. PHI profiles of number of intercepts, on the horizontal axes, in each height interval, on the vertical axes, in dm. On the left are Leptospermum scoparium at 14 and 30 years; for 50 and 70 years the graphs show a combination of Leptospermum scoparium, Kunzea ericoides and Erica lusitanica. On the right are profiles for N. solandri var. solandri. The percentage within each graph is the S%CS for each species or group of species.

Fig. 5. PHI profiles of number of intercepts, on the horizontal axes, in each height interval, on the vertical axes, in dm. for N. solandri var. solandri. The percentage within each graph is the S%CS for each species or group of species.

Fig. 6A. The variation of two indices of species diversity, of number of vascular plants with time: S, species richness, and antilog H′, the Shannon-Wiener quantity weighted diversity index (x 10 for comparability).

Fig. 6B. The relation between antilog H′ and Crown Space Percent (CS%), a measure of stand foliar density. There is low quantity-weighted diversity, H′, and low CS% in the pure N. solandri var. solandri stand, there is greater spread of CS% values in the Leptospermum stands. The quantity-weighted diversity index is highest in the densest stands, of high CS%, in communities at 14 and 70 years, which are in transition from one vegetation type to another.